1. Field of the Invention
The present invention relates to an angular-position magnetic-sensor device. It is applicable in particular but not exclusively to motorized butterfly valves (ETC), which require a very stable signal at the beginning of travel and a fairly large travel (larger than 90°) with excellent linearity.
2. Description of the Related Art
In general, contactless systems comprise substantially devices made from angular-position magnetic sensors. In all such devices, the measured signal must be as linear as possible according to angular position.
In order to make both the invention and the angular-position magnetic-sensor devices more comprehensible, it is important to understand the “zero gauss” concept.
The position delivering a signal equal to zero gauss corresponds to the point of high stability. In fact, the variation of induction is measured by means of an intelligent and programmable Hall-effect sensor of ASIC type. This sensor is capable of compensating for the effect of temperature (programming of a thermal coefficient) on the magnetic system. Such temperature compensation is not perfect, and therefore the only position for which this error has no influence is a position at which induction is zero.
French Patent Application No. 2670286 and European Patent Application No. 0665416, filed in the name of the Applicant, disclose a device in which the angular-position measurement function is achieved by variation of the magnetic induction, but propose a maximum travel of ±90° around the zero-gauss position. For sensors of the type presented in the aforesaid two patents, this travel is reduced to ±70° around the zero-gauss position in an attempt to obtain excellent linearity (typically ±0.5%). Since ETC applications generally necessitate a total travel of close to 90°, two options are possible:                the use of a symmetric angular range, such as [−45°; +45°], or        the use of the angular range [−20°; +70°], so as to approach zero gauss.        
Thus it is not possible in any of the cases to combine the following three characteristics:                a zero-gauss starting point (high-stability point),        a usable angular travel greater than or equal to 90°,        excellent linearity (±0.5%) over the chosen angular range.        
In fact, if the interval [−20°; +70°], for example, is used, the starting point (−20°) does not operate at zero gauss but exhibits an offset, which is problematic for applications that necessitate a very stable starting point.
In the prior art there is also known U.S. Pat. No. 5,861,745, which describes a contactless detection device comprising a rotor and a stator. In the stator there is disposed a Hall element in an air gap having the form of a slit, and in the interior wall of the rotor there are disposed two annular magnets having mutually opposed magnetic polarities.
Within the scope of use of the angular sensor over the maximum travel (240°) and/or for applications of the ETC type, however, only two structures function theoretically, those illustrated in FIGS. 3 and 4, which are composed respectively of an interior stator configuration and an exterior stator configuration. These two devices exhibit the following characteristics: three pieces for the stator and two magnets that are magnetized radially, one (interior radial) over an angular extent of 120° and the other (exterior radial) over an angular extent of 240°.
At present, however, it is not technically feasible to magnetize a magnet correctly over an angular extent of 240°. This degradation of performances leads inevitably to poor linearity and, as it happens, linearity is the primary criterion for sensors of this type.
Finally, there is also known International Patent Application No. WO 0120250, which describes a measuring device for contactless detection of a relative motion, comprising 3 stator poles of approximately 120° by making of a hole in the large stator, with the objective of decreasing the leakage flux when the transition of the magnets is located in the configuration illustrated in particular in FIGS. 22, 23, 24a and 24c of the said application. Thus the device of that patent application does not permit operation in the angular phase [−120°; +120°], because the presence of the said hole induces an “accident” in the measured induction for the zone close to zero.